1. Field of the Invention
This invention relates to polymer blend, hollow fiber membranes having improved mechanical properties by alteration of the thermodynamic phase equilibrium of the concentrated polymer blend spinning solution through the incorporation of organic or inorganic complexing agents.
2. Description of the Related Art
The science of polymer blend miscibility and the prediction of the types of phase diagrams for polymer blends in solutions are quite complex. Unusual two-peaked coexistence curves and phase diagrams with a tendency toward greater miscibility at intermediate temperatures are reported in the literature for different polymer blends. The following articles and textbooks depict the complexity of the theory of polymer blend miscibility and provide several examples of different thermodynamic phase diagrams for polymer blends:
R. L. Scott, J. Chem. Physics, Vol. 17, p. 279 (1949);
D. R. Paul and S. Newman, Polymer Blends, Vols. 1 and 2, Academic Press, San Francisco, London (1978);
0. Olabasi et al., Polymerxe2x80x94Polymer Miscibility, Academic Press, NY (1979); and
Pierre-Gillesde Gennes, Scaling Concents in Polymer Physics, Chapter 4, Sections 4.1 and 4.2, Cornell University Press (1979).
Polymer blends which are molecularly compatible in the solid state at temperatures below the glass transition temperature of the blend might exhibit a thermodynamic phase equilibrium consisting of a coexistence curve in the solution state. The coexistence curve is the locus of the critical solution temperature (CST) as a function of the blend composition.
A particular blend solution might exhibit a coexistence curve for the higher critical solution temperature (HCST), or for the lower critical solution temperature (LCST), or for both the HCST and the LCST. For any-specific blend composition, the blend solution is two-phase if the temperature is below the HCST or if the temperature is above the LCST (assuming that the phase diagram contains coexistence curves both for the HCST and LCST). For that type of phase diagram, the blend solution is single phase for any specific blend composition if the blend solution temperature is above the HCST and below the LCST.
U.S. Pat. No. 5,047,487 issued to Camargo et al. discloses that ULTEM 1000, a polyetherimide available from GE, and MATRIMID 5218, a phenylindane-containing polyimide available from Ciba, are molecularly compatible in the solid state. The molecular scale compatibility of the two polymers over the entire blend composition range was characterized by Camargo et al. utilizing the technique of Differential Scanning Calorimetry (DSC). The ULTEM/MATRIMID blends exhibit a single glass transition temperature (Tg) located in between the Tg of the individual blend components over the entire blend composition range, which indicates molecular-scale blend miscibility.
U.S. Pat. No. 5,085,676 issued to Ekiner et al. discloses solution spinning of hollow fiber membranes from concentrated ULTEM/MATRIMID blend solutions. U.S. Pat. No. 5,443,728 discloses membranes prepared from blends of polyetherimide and phenylindane-containing polyimides.
In prior practices, blend solutions have been stored above the HCST to ensure phase homogeneity. This may unfortunately result in polymer degradation reactions due to sensitivity of the blend polymers to high temperature exposure for prolonged times. The degradation of the blend polymer adversely affects the final product properties.
On the other hand, storage at temperatures below the HCST results in a two-phase solution. With prior methods, additional processing steps for heating and mixing of the two-phase blend solution were needed for formation of a homogenous blend solution prior to the final processing step. A two-phase blend polymer phase morphology in the solution state would also adversely affect the fiber spinning process continuity and the final product properties.
ULTEM and MATRIMID polyimide solutions are particularly susceptible to molecular weight degradation reactions in the solution state when exposed to elevated temperatures for prolonged times.
It is, therefore, an object of the present invention to provide a polymer blend that does not suffer from the disadvantages mentioned above. In particular, it is an object of the present invention to depress the HCST of a polymer blend solution in order to enhance its phase stability during low temperature storage prior to processing. It is a further object of the present invention to depress the HCST of the ULTEM/MATRIMID blend solution formulations in order to permit storage of the solutions at lower temperatures without phase separation prior to solution spinning into hollow fiber form.
These and other objects of the invention will become apparent in light of the following specification, the figures, and the claims appended hereto.
In one of its composition aspects, the present invention relates to a polymer blend comprising a plurality of polymers and a CST adjustment agent. As used herein, a CST adjustment agent is an organic or inorganic complexing agent which enhances the miscibility of two or more polymers in solution.
Polymer blends particularly suited for use in the present invention include polyimide blends such as a blend of polyetherimide and phenylindane-containing polyimide. In a preferred embodiment, the polymer blend comprises two commercially available polymers, ULTEM and MITRIMID. Preferably, the polymer blend comprises between about 80% and 95% by weight of polyetherimide and the balance phenylindane-containing polyimide.
The CST adjustment agent is preferably an alkali or alkaline earth metal halide such as ZnCl2, CaBr2, and LiCl.
The CST adjustment agent can also be organic; in which case, triethylamine is particularly preferred.
It has been surprisingly found that through the use of a CST adjustment agent according to the present invention, the HCST of a polymer blend solution is lowered by at least 10xc2x0 C. relative to the same blend solution without the addition of a CST adjustment agent.
In another of its composition aspects, the present invention relates to a polymer blend which has been annealed at elevated temperature. It has been surprisingly discovered that annealing a polymer blend solution at elevated temperature has the effect of lowering the HCST of the solution. The precise annealing time depends on the polymer blend solution employed. Such times can be determined by routine optimization by those skilled in the art for the particular polymer blend solution. The polymer blend solution is preferably annealed at an elevated temperature between 50xc2x0 and 140xc2x0 C., and more preferably between 700xc2x0 and 100xc2x0 C. In this embodiment, the polymer blend may or may not contain a CST adjustment agent.
In one of its method aspects, the present invention relates to a method for modifying the CST of a polymer blend solution by combining the polymer compounds of the polymer blend together with a solvent and a CST adjustment agent. The resulting CST-modified polymer blend solution may be used to form hollow fiber membranes having improved mechanical properties.
In another of its method aspects, the present invention relates to a method for lowering the CST of a polymer blend solution by annealing the solution at elevated temperature.
In yet another of its method aspects, the present invention relates to a method for the separation of gases from a mixture, preferably air. The method includes the step of bringing the gas mixture into contact with a composite hollow fiber membrane formed from a polymer solution containing a CST adjustment agent in accordance with the present invention, at an elevated pressure to preferentially permeate at least one component of the gas mixture to produce at least one gaseous product.